This invention relates to an ion beam irradiation apparatus suitable for use in cancer radiotherapy.
Ion beam irradiation is widely employed for treating tumors, and in particular, tumors that are not easily accessible to surgery. The ion beam is produced by a particle accelerator, which is directed toward the tumor and controlled in so that the ions lodge in the tumor cells without damaging the surrounding tissue. The size of the tumor often exceeds the diameter of the beam. It is therefore necessary to spread the beam in such a way that the beam will deliver a substantially uniform ion dose to an area large enough to cover the entire tumor.
An efficient method of spreading an ion beam is to wobble the beam by a pair of deflection magnets, where one magnet deflects the beam in the x-direction and the other magnet deflects the beam in the y-direction. A prior-art apparatus employing this method has been used at the Bevalac facility at the Lawrence Berkeley Laboratory of the University of California. as described in IEEE Transactions on Nuclear Science, Vol. NS-32, No. 5, October 1985, and in Medical Physics, Vol. 14, No. 5, September/October 1987. The power supply of the prior-art apparatus, which includes a three-phase alternator and an autotransformer, supplies sinusoidal current waveforms offset by 90.degree. C. in phase to the x- and y- deflection magnets. This apparatus causes the beam to be deflected in a circular path, and the radius of the beam may be adjusted by changing the amplitude of the waveforms. By having the beam trace three concentric circular paths it is possible to "paint" having a circular field 30 cm in diameter for example with a substantially uniform radiation distribution.
One problem with the prior-art apparatus is that tumors, particularly difficult tumors which require radiotherapy, are rarely circular in shape. The circular field must therefore be partially masked to avoid bombarding healthy tissue. However, a large percentage of the ions emitted from the accelerator is wasted and is increased when the circular field is partially masked the treatment time needed to achieve the required dose.
Another problem is that the circular field painted by the prior art apparatus is always centered on the beam axis. and cannot be moved. To irradiate different tumor sites it is therefore necessary to move the patient, a procedure which requires an unnecessary amount of time. The treatment time is an important factor in view of both patient discomfort and the high cost of the particle accelerator.